Readme update

[scolsen]

Adds documentation on enabling folding using g:markdown_folding. Introduces some parallel grammar and fixes minor typos. Adds some section headers and styling for readability.

[anthony-geoghegan]

I think these are all useful commits and it would be great to see this pull-request merged.

I’d suggest a few further changes:

• wrap the long lines (those beginning with Note)
• change the Fenced Codeblock Highlighting heading to Fenced code block highlighting – “code block” is usually written as two words in Markdown documentation sources (including this file).

Also, the top-level heading of the initial README uses the style of capitalising only the first word (and proper nouns such as “Markdown”) so it would make sense to be consistent in using this style for all headers.

[scolsen]

@anthony-geoghegan thanks for the bump! Incorporated your suggestions.

[anthony-geoghegan]

Nice one, @scolsen Let's hope it gets merged. I recently opened a pull-request, myself but I'm guessing Tim has more important things on his mind in these troubled times.